Model based detection and compensation of glitches in color measurement systems

ABSTRACT

A color sensor monitors the output of a color producing process and produces a signal representative of a color produced by the color producing process. The signal can be used as feedback signal to control the process. Occasionally, the color sensor signal includes a component representing a transient error. A system model of the color producing process is used to predict reasonable sensor signals. A comparison of the sensor signal with the predicted sensor signals is used to determine if the sensor signal is reasonable. If the sensor signal is unreasonable, a substitute signal is used as the feedback signal to the control process. The substitute signal can be a predicted sensor signal or a signal based on historical system performance data.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to the art of color measurement. Morespecifically the invention is related to a method and apparatus for thedetection and filtering of transient errors from a signal produced by acolor measurement device. The method includes the substitution of areasonable signal for color measurement signals that are outsidereasonable levels. A system model or historical data are used todetermine a reasonable signal level for a given color. The inventionwill be described in reference to a color reprographic environment suchas, for example a color xerographic environment. However, the inventionis applicable to any environment where color is a useful measure of aprocess or plant. For example, the invention finds application inmonitoring the dying of textiles, plant hydration, food processing andindustrial control applications, such as, for example, coil coating.

2. Description of Related Art

In order to provide the best possible color reproduction, reprographicdevices have been developed that include color sensors. For example,spectrophotometers have been included in reprographic environments, suchas, for example color xerographic environments. The color sensorsprovide color feedback signals for color control systems. The colorcontrol systems use the feedback signals to selectively adjust controlparameters and/or color compensation techniques to maintain optimumcolor reproduction performance.

For example, a color control system may use a color feedback signal toadjust the shape or values associated with a tone reproduction curve(TRC). Tone reproduction curves are known in the art of colorreproduction. Tone reproduction curves relate a given color request orinput, to an amount of colorant or output signal required to produce therequested color.

In order to maximize a number or percentage of high quality copiesproduced in a given reproduction run, the color control systems aretuned to respond quickly to system changes reported by the color sensor.As a result, noise or sensor glitches can cause significant controlsystem disturbances. Such disturbances can affect the quality of anunacceptably large number of print job copies. Additionally, unnecessarymachine shut downs and service calls are sometimes made in response tothe print quality problems resulting from momentary sensor errors.

One solution to this problem is to detune the color control system.Detuning the system would slow system response, thereby reducing thesignificance of any short-lived color sensor transient. However,detuning the system would also slow system responses to less transientevents and perhaps increase following errors in general.

Another solution to this problem might be to add one or more backup orredundant color sensors to the system. For example, a plurality of colorsensors, sensing the same color or image portion could be polled. Themost reasonable measured color sensor signal could then be treated asthe correct or preferred sensor signal. However, this approach hasseveral drawbacks. For example, color sensors are expensive.Additionally, multiple sensors may not fit in a particular system.

Instead of detuning the color control system or installing backupsensors, it is preferable to simply log or count transient errors and tocontinue printing based on best available information. For example, asystem model can be used to temporarily replace the sensor signal. Asystem shut down and/or service call need only occur if, for example,the transient errors occur at an unacceptably high rate.

BRIEF SUMMARY OF THE INVENTION

To those ends, a method operative to process transient errors producedin a color measurement system monitoring a color producing process hasbeen developed. The method comprises implementing a model of the colorproduction process, monitoring an input to the color production process,predicting an expected color signal based on the model and the monitoredinput, measuring an output color produced by the color producing processto produce a measured color signal, comparing the measured color signalto the expected color signal to produce a color error value, and,selectively replacing the measured color value based on the color errorvalue.

For example, selectively replacing the measured color value includesreplacing the measured color value with a predicted color signal basedon the expected color signal. For instance, if the color error value islarge, because the measured color value is unreasonable or verydifferent than the predicted color value, then the measured color valuemay be replaced with the predicted color value.

One embodiment of the invention is a method for calibrating a colorreproduction device. The method includes producing an image with thereproduction device in response to an input signal requesting theproduction of a target color, measuring with a sensor, a color of theproduced image, to generate a measured color signal value, calculatingan estimated color signal value based on the input signal, validatingthe measured color signal value by comparing it to the estimated colorsignal value, selecting a preferred color signal value from among atleast the measured color signal value, and the estimated color signalvalue, based on the validity of the measured color signal value,determining an error between the preferred color signal value and thetarget color, and, selectively adjusting parameters of a control systemof the color reproduction device to minimize the determined error forsubsequently produced images.

An exemplary system operative to filter transient errors from a colormeasurement signal includes a color producing process, a model of thecolor producing process, the model and the process operative to receivean input and, based on the input, produce a model color signal and aprocess output, respectively. The system also includes a color sensoroperative to produce a measured color signal representative of theprocess output color, a preferred signal selector operative to select apreferred signal from among at least the model color signal, and themeasured color signal, and, a signal consumer, operative to receive thepreferred signal from the preferred signal selector. For example, thesignal consumer is a system controller, wherein the preferred signal isoperative as a feedback signal.

One advantage of the present invention relates to providing a filteredcolor measurement signal.

Another advantage of the present invention resides in a reduction orelimination of undesirable control system perturbations. The reductionin control system perturbations results in an increased system outputquality and an increase in print job yield.

Yet another advantage of the present invention is found in an ability tomaintain a high frequency response with regard to non-transient systemchanges.

Still other advantages of the present invention will become apparent tothose skilled in the art upon a reading and understanding of the detaildescription below.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various procedures and arrangements of procedures.The drawings are only for purposes of illustrating preferredembodiments, they are not to scale, and are not to be construed aslimiting the invention.

FIG. 1 is a flow chart outlining a method of filtering a color sensorsignal.

FIG. 2 is a flow chart of a system including an embodiment of the methodof FIG. 1.

FIG. 3 is a block diagram of a system operative to perform the method ofFIG. 1.

FIG. 4 is a block diagram of a reprographic system operative to performthe method of FIG. 1.

FIG. 5 is a graph illustrating the color signal glitch and the result ofusing the method of FIG. 1 to filter out the glitch.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a method 108 for filtering transient errors from acolor measurement signal includes using a model to determine when acolor signal contains noise or a glitch. For instance, a color sensor isused to monitor the output of a system that produces, or effects, acolor. The color sensor generates a color measurement signal.Preferably, the measurement signal is representative of the systemoutput. However, the color measurement signal occasionally includesnoise or measurement glitches. The noise or measurement glitches are tobe filtered out. The filtering method 108 includes the selection and/orimplementation 112 of a model that describes the color producing oreffecting system. The system has an input, such as, for example, asignal representing a requested color. The system input is monitored 114or recorded. A copy of the input is entered into the system model. Thesystem model generates 118 a predicted output color or predicted colorsignal. Optionally, and/or selectively, the color measurement signal,generated by the color sensor, and/or other feedback signals, is used tocontinuously modify or update system model parameters. Of course, thecolor sensor measures 122 the actual color output of the system and, asmentioned above, produces the color measurement signal. The colormeasurement signal and the predicted color signal are compared 126 todetermine if the color measurement signal is reasonable or within areasonable range of the predicted color signal.

If 126 the measured color signal is within a reasonable range of thepredicted color signal then the measured color signal is deemed to becorrect and the measured color signal is used in further processing 130.For example, the measured color signal is used as a feedback signal. Thefeedback signal is used to close a control loop and/or to update systemmodel parameters. The measured color signal may also be logged orentered into a database to contribute to an historical performancedatabase. The historical performance database may be used as analternate predictor of system output.

For example, the measured color signal is stored in the database inrelation to the monitored system input signal. Regression techniques areused on a plurality of such entries to determine parameters orcoefficients of a system descriptive polynomial. The polynomial can beused to generate or predict system outputs based on system inputsignals.

If 126 the measured color signal is outside a reasonable range from thepredicted color signal, an error counter is incremented 134. The errorcounter is incremented 134 in order to keep track of a number of, or arate at which, transient errors or glitches occur. While an occasionaltransient error can and should be ignored for feedback or historicalperformance database purposes, frequent or persistent transient errorevents may indicate the presence of a significant problem in the system.Therefore, in an error counter evaluation 138 a determination is made asto whether transient errors have been occurring too frequently or if atotal number of the transient errors is too high. If 138 the results ofthe evaluation warrant it, provisions are made to repair the system 142.For example, an alarm message in displayed requesting that a servicecall be made. Alternatively the system itself may send an electronicmessage, such as an email or a phone call, requesting diagnostic andrepair services. Even if repair services are requested, system operationmay continue. Certainly, if transient errors have been occurring at atolerable rate, processing continues. A signal substitution 146temporarily replaces the color measurement signal with a preferredreasonable value. For example, the predicted color signal is temporarilyused for the purposes of further processing. Alternatively, a valuederived from the historical database is used. Whatever value is selectedto be the preferred color signal, that value is used in further systemprocessing 150. For example, the preferred 146 color signal may be usedin place of the color measurement signal for the purpose of closing acontrol loop. Of course, the phrase—closing a control loop—is usedloosely here. The preferred 146 color signal is delivered to a controlsystem that expects and requires a feedback signal. However, since thepreferred 146 color signal is mathematically or statisticallydetermined, and not a real measurement, the preferred 146 color signalis not a closed loop feedback signal in the strictest sense.Nevertheless, the preferred 146 color signal is expected to be a closerrepresentation of current system performance that is the glitch ortransient containing, measured color signal. In this respect, thetemporary substitution of the preferred color signal for the measuredcolor signal filters the glitch or transient error from the measuredcolor signal.

The method 108 for filtering transient errors from a color measurementsignal can be used in many industries and devices. For example, themethod 108 for filtering transient errors can be used in conjunctionwith a textile manufacturing plant. The color resulting from theapplication of dyes or combinations of dyes can be monitored andcontrolled through the use of one or more color sensors that producecolor measurement signals. The color measurement signals can bebeneficially filtered using the method 108 for filtering transienterrors. Similarly, the processes of a green house can benefit from themethod for filtering transient errors from a color signal. The amount ofwater, a plant receives can affect, for example, the color of the plantsleaves and/or flowers or fruits. Plant hydration can be controlledthrough the use of a feed back signal from a color sensor. Such a systemmay benefit from the use of the method 108 for filtering transienterrors from a color measurement signal. The method 108 for filteringtransient errors from a color measurement signal is also beneficiallyapplied to reprographic environments, such as, for example, colorxerographic, ionographic, ink jet, and other reprographic environments.

For example, referring to FIG. 2, a reprographic process 208, such as acolor xerographic reprographic process includes an embodiment 212 of themethod 108 for filtering transient errors from a color measurementsignal. A system control process 216 communicates with sub-processes ofthe embodiment 212. The control process receives or generates (notshown) an input signal representing a target color to be printed. Forexample, the control process receives a target color from an imagesource such as a scanner or a computer network. Alternatively, thecontrol process 216 generates a target color input signal of its own.For example, the control process generates an input signalrepresentative of a calibration color patch. For instance the colorpatch is a gray color patch composed of a mixture of several colorants(e.g. cyan, magenta, and yellow). The system input is monitored 220. Themonitoring process 220 fetches a copy of the input signal and passes itto a modeling process 224. Optionally, or selectively, model parametersare updated 228. For example, the control system database may be updatedas a result of measured color signals previously received from a colorsensor. Preferably, the modeling process 224 generates a predicted orexpected color signal in the same units or color space as the colorsensor operates in. For example, where the color sensor generates ameasured color signal in units of L*a*b* of the CIELAB (CommissionInternationale de l'Eclairage) color space, the modeling processpreferably generates a predicted color signal in the same L*a*b* units.The modeling process can take advantage of any applicable model. Forexample, the modeling process 224 can use a Neugebauer model, such as,for example a refined parameterized Neugebauer model. Alternatively, themodel used by the molding process can be a multidimensional numericalmodel. For example, the model can be a multidimensional numerical modelbuilt using interpolation on data obtained through input-outputexperiments. In a further alternative the model is an on-linestatistical parameterized model estimated using real-time systemidentification techniques.

In any event, preferably contemporaneous with the modeling process 224,the control system sends commands to a rendering device, such as, axerographic, ionographic, or ink jet print engine, to render or produce229 a color in response to the input signal. As mentioned above thecolor can be part of a source image provided by or indicated by a systemuser, or the color can be part of an internally generated or retrievedcalibration image.

The rendered color is measured 232 by a color sensor. For example, therendered color is measured by spectrophotometer. The color sensorgenerates a measured color signal in response to the rendered color. Asmentioned above, preferably the measured color signal is calibrated inthe same units as the predicted color signal.

The measured color signal and the predicted color signal are compared236 to determine if the color measurement signal is reasonable or withina reasonable range of the predicted color signal. For example, whenoperating in the CIELAB color space, a deltaE value is computed. ThedeltaE value is a measure of a distance between the two points (themeasured and predicted color signals) in L*a*b* space.

The magnitude of the deltaE value is compared 240 to a deltaE thresholdvalue. If the measured color signal is within a reasonable range of thepredicted color signal (i.e. less than the deltaE threshold), then themeasured color signal is deemed to be correct and the measured colorsignal is used in further processing 244.

For example, in one xerographic environment, a threshold value of 20deltaE appears to be satisfactory. A useful method for selecting atleast an initial threshold value for a particular reprographic system isto compute the centroid of a color gamut of the associated renderingdevice. For example, input-output experiments are run to determine therange of colors the rendering device can produce. Once the color gamutof the rendering device is established, the deltaE of the boundary pointfurthest from the centroid can be determined. This boundary point deltaEvalue is suitable as a first threshold value. Of course, actual filterperformance may warrant adjustments to this first threshold value.Furthermore, it may be useful to vary the threshold value as a functionof, for example color space location. For instance, one threshold valuemay be appropriate for boundary colors, while another threshold valuemay be appropriate for colors near the centroid.

As mentioned, if the measured color signal is within a reasonable rangeof the predicted color signal then the measured color signal is deemedto be correct and the measured color signal is used in furtherprocessing 244. For example, the measured color signal is used as afeedback signal to the system control process 216. The feedback signalis used to close a control loop and/or to update system modelparameters. The measured color signal may also be logged 244 or enteredinto a database to contribute to an historical performance database. Asmentioned above, the historical performance database may be used as analternate predictor of system output or as a source of a preferred colorsignal.

If the measured color signal is outside a reasonable range from thepredicted color signal an error counter is incremented 248. The errorcounter is incremented 248 in order to keep track of a number of, or arate at which, transient errors occur. While an occasional transienterror can and should be ignored for feedback or historical performancedatabase purposes. Frequent or persistent transient error events mayindicate the presence of a significant problem in the color sensor.Therefore, in an error counter evaluation 252 a determination is made asto whether transient errors have been occurring too frequently or if atotal number of transient errors is too high. If the results of theevaluation warrant it, provisions are made to repair the color sensor256. For example, an alarm message in displayed requesting that aservice call be made. Alternatively the system itself may send anelectronic message, such as an email or a phone call, requestingdiagnostic and repair services. Even if repair services are requested,system operation may continue. Certainly, if transient errors have beenoccurring at a tolerable rate, processing continues. A signalsubstitution 260 temporarily replaces the color measurement signal witha preferred reasonable value or color signal. For example, the predictedcolor signal is temporarily used for the purposes of further processing.Alternatively, a value derived from the historical database is used.Optionally, the preferred signal is logged 244 along with an indicationof the source of the preferred signal (model, historical data, etc).Whatever value is selected to be the preferred color signal, that valueis passed to the system control process 216 and used in further systemprocessing. For example, the preferred color signal may be used in placeof the color measurement signal for the purpose of closing a controlloop. As mentioned above, the phrase—closing a control loop—is usedloosely here.

Referring to FIG. 3, a system 308 including a filtered color sensor 312is operative to produce a color. The filtered color sensor 312 isoperative to monitor the produced color. The system 308 includes a plantor system controller 316 and a plant or process 320. The plant or system316 can be any machine, factory, process, system or device that producesa color as a part of its output or as its output as a whole. Forexample, the plant can be a pigment manufacturing machine, a greenhouse, a textile mill, a food processing plant, or any other system thatproduces a color or a colored product or output. The filtered colorsensor 312 portion of the system 308 includes a color sensor 324, aplant model 328, a signal comparer 332, and a preferred signal selector336.

The plant controller 316 receives a set point or target 340 related to adesired output color. The controller 316 translates that set point ortarget 340 into a plant input 344 or series of plant inputs or commandsthat are delivered to both the plant 320 and the plant model 328. Theplant 320 responds to the input 344 by producing output 348. Ideally,the output includes a color that exactly matches a color indicated bythe set point or target 340. However, due to imperfections in material,machinery wear, and drifts in process parameters or parameters affectedby environmental factors such as, for example, temperature and humidity,the output may not have a color that is an exact match to the colorindicated by the set point or target 340.

The model 328 includes mathematical equations or functions thatrepresent the processes of the plant 320. The model is any system modelthat can be adapted to accurately represent the plant. Exemplary modelsinclude a refined parameterized Neugebauer model, a multidimensionalnumerical model built using interpolation on data obtained through input344-output 348 experiments, and an online parameterized statisticalmodel estimated using real-time system identification techniques.

The model responds to the input 344 by calculating or predicting thecolor of the output, thereby generating a predicted or expected colorsignal output 352. The prediction is based on the selected form or typeof the model, and on parameters or coefficients related to the model.Preferably, the parameters and coefficients of the model are updated ona regular basis, based on monitored inputs 344 and outputs 348 asmeasured by the color sensor 324.

The color sensor 324 is, for example, a spectrophotometer. The colorsensor monitors the color of the plant output 348 and produces ameasured color signal 356 in response to it. The measured color signal356 and a reference signal, such as, for example, the predicted colorsignal 352 are delivered to both the signal comparer 332, and thepreferred signal selector 336. The comparer 332 compares the signals andproduces a comparison signal 360 that represents a measure of thedifference between the measured color and the predicted color. Thecomparison signal 360 is delivered to the preferred signal selector 336.

The preferred signal selector 336 evaluates the comparison signal inlight of a comparison signal threshold 364. The comparison signalthreshold 364 is a limit on a magnitude of the comparison signal 360.For example, where the comparison signal 360 represents a differencebetween the measured color signal 356 and the predicted color signal352, the comparison signal threshold 364 is a limit on the magnitude ofthat difference. If the comparison signal 360 is below the comparisonsignal threshold 364, then the preferred signal selector 336 selects themeasured color signal 356 to be the preferred color signal 368. If thecomparison signal 360 is above the comparison signal threshold 364, thenthe preferred signal selector 336 selects the predicted color signal 356to be the preferred color signal 368. Alternatively, the preferredsignal selector may select a signal from another source (not shown),such as, for example, a regression curve derived from historical plantperformance data stored in a historical performance database (notshown).

In any event, the preferred color signal 368 is delivered to the plantcontroller 316 for further processing. For example, the plant controlleruses the preferred color signal as a feedback signal. Such a feedbacksignal is used to close a control loop and/or to modify the way thesystem or plant controller 316 translates set points or targets 340 intoplant input signals 344.

Referring to FIG. 4, in reprographic environment, such as, for example,a color xerographic environment the system 308 is a color reprographicsystem 408. The color reprographic system includes a filtered colorsensor 412. The filtered color sensor 412 is operative to monitor aproduced color. The system 408 includes a printer or color controller416 and a rendering device 420 such as, for example, a xerographic,ionographic or ink jet printer. Xerographic printers are known in theart to include a fuser, a developer, and an imaging member. The filteredcolor sensor 412 portion of the reprographic system 408 includes a colorsensor 424, a model of the rendering device 428, a signal comparer 432,and a preferred signal selector 436.

The color controller 416 receives a set point or target color 440related to a desired output color. The target color may be part of asystem user specified image or a system calibration target generated by,for example, a system calibrator (not shown). The controller 416translates that set point or target 440 into a printer input 444 that isdelivered to the printer 420 and the printer model 428.

Preferably the printer and printer model 428 accept input in the sameunits of measure. For example, typically a printer accepts input inmachine dependent form. For example, a printer may accept input in theform of request for specific amounts of colorants. Cyan, Magenta, Yellowand Black (CMYK) are the names of common colorants. Therefore, printersaccept inputs in the form of CMY or CMYK values. In such systems it ispreferable that the printer model 428 also accept input in the form ofCMY or CMYK values. The printer or rendering device 420 responds to theinput 444 by producing an output image 448. Ideally, the output image448 is of a color that exactly matches a color indicated by the setpoint or target 440. However, due to imperfections in material,machinery wear, and drifts in process parameters or parameters affectedby environmental factors such as, for example, temperature and humidity,the output image may not exactly match to the color indicated by the setpoint or target 440.

The printer model 428 includes mathematical equations or functions thatrepresent the processes of the printer 420. The model is any systemmodel that can be adapted to accurately represent the printer. Exemplarymodels useful in modeling printers include a refined parameterizedNeugebauer model, a multidimensional numerical model built usinginterpolation of data obtained through input 444-output 448 experiments,and an online parameterized statistical model estimated using realtimesystem identification techniques.

The model responds to the input 444 by calculating or predicting thecolor of the output, thereby generating a predicted color output 452.The prediction is based on the selected form or type of the model, andon parameters or coefficients related to the model. Preferably, theparameters and coefficients of the model are updated on a regular basisbased on monitored inputs 444 and outputs 448 as measured by the colorsensor 424.

The color sensor 424 is, for example, a spectrophotometer. The colorsensor monitors the color of the output image 448 and produces ameasured color signal 456 in response to it. The measured color signal456 and the predicted color signal 452 are delivered to both the signalcomparer 432, and the preferred signal selector 436.

Preferably, the measured color signal 456 and the measured color signal456 are in the same units. For example, preferably, the color sensor 424produces a measured color signal in a machine independent form such as,for example, L*a*b*, XYZ or Luv. In order to avoid expensive (inprocessing time) and error prone conversions, it is preferable that theprinter model 428 generate a predicted color signal in the same L*a*b*,XYZ or Luv units as the color sensor 424.

The comparer 432 compares the measured 456 and predicted 452 colorsignals and produces a comparison signal 460 that represents a measureof the difference between the measured color and the predicted color.The comparison signal 460 is delivered to the preferred signal selector436.

The preferred signal selector 436 evaluates the comparison signal inlight of a comparison signal threshold 464. The comparison signalthreshold 464 is a limit on a magnitude of the comparison signal 460.When, the measured and predicted color signals are in units of L*a*b*space, the comparison signal threshold 464 can be in units of deltaE.Where the comparison signal 460 represents a difference between themeasured color signal 456 and the predicted color signal 452, thecomparison signal threshold 464 is a limit on the magnitude of thatdifference. If the comparison signal 460 is below the comparison signalthreshold 464, then the preferred signal selector 436 selects themeasured color signal 456 to be the preferred color signal 468. If thecomparison signal 460 is above the comparison signal threshold 464, thenthe preferred signal selector 436 selects the predicted color signal 456to be the preferred color signal 468. Alternatively, the preferredsignal selector may select a signal from another source (not shown),such as, for example, a regression curve derived from historical plantperformance data stored in a historical performance database (notshown).

In any event, the preferred color signal 468 is delivered to the colorcontroller 416 for further processing. For example, the plant controlleruses the preferred color signal as a feedback signal. Such a feedbacksignal is used to close a control loop and/or to modify the way thesystem or plant controller 416 translates set points or targets 440 intoprinter input signals 444.

In summary, referring to FIG. 5, in operation the method 108 forfiltering a color signal 508, performs a filtering operation bydetecting a glitch 512 or transient error in the signal and replacing itwith a more likely or more reasonable value 516. The more reasonablevalue may come from a system model or from historical performance dataabout the system. The glitch is detected, by comparing the color signalto the output of a system model or to recent historical data reflectiveof previous system outputs related to similar system inputs or setpoints. Filtering or removing glitches from the color signal providesimproved system stability, while allowing for critically damped or underdamped system tuning.

The invention has been described with reference to particularembodiments. Modifications and alterations will occur to others uponreading and understanding this specification. While the invention has,for the most part, been described in terms of monitoring rendered colorsgenerated by a printing device, the invention can be used to monitor anyprocess that produces a color. For example the invention can be appliedto agricultural applications to monitor a color of plants to control theamount of water, light or fertilizer they receive. The invention can beused to monitor the mixing of pigments. For example, the invention findsapplication in the control of the mixing of dyes or the mixing ofpaints. Those skilled in the art will understand how to modify exemplaryembodiments to apply them these and other applications. It is intendedthat all such modifications and alterations are included insofar as theycome within the scope of the appended claims or equivalents thereof

1. A method of processing transient errors produced in a colormeasurement system monitoring a color producing process, the methodcomprising: implementing a model of the color producing process;monitoring an input to the color producing process; predicting anexpected color signal based on the model and the monitored input;measuring an output color produced by the color producing process toproduce a measured color signal; comparing the measured color signal tothe expected color signal to produce a color error value, and;selectively replacing the measured color signal based on the color errorvalue.
 2. The method of processing transient errors of claim 1 whereinselectively replacing the measured color signal comprises: replacing themeasured color signal with a predicted color signal based on theexpected color signal.
 3. The method of processing transient errors ofclaim 1 further comprising: storing a measured color valuerepresentative of the measured color signal in association with themonitored input.
 4. The method of processing transient errors of claim 1wherein selectively replacing the measured color signal comprises:replacing the measured color signal with an historical color signalbased on an historical value related to the monitored input.
 5. Themethod of processing transient errors of claim 1 wherein implementing amodel of the color production process comprises: selecting at least oneof a refined parameterized Neugebauer model, a multidimensionalnumerical model and an on-line statistical parameterized modelrepresentative of the color producing process.